address review: pre-P shadow fallback and safer notification star

- draw the play halo as an offset copy below API 28, where path
  shadow layers are ignored, and skip the inner shadow there too
- add a repository toggleStar that reads and writes under the star lock,
  so fast double taps on the notification cannot desync
- reset the star button when a track starts, so a failed source lookup
  cannot leave the previous song's state showing

app/src/main/kotlin/com/subtracks/data/repo/LibraryRepository.kt

@@ -209,30 +209,47 @@ class LibraryRepository(
         starScope.launch { setStar(type, id, starred) }
     }
 
+    fun toggleStar(
+        type: StarType,
+        id: String,
+    ) {
+        starScope.launch {
+            starLock.withLock {
+                val sourceId = sourceRepository.activeSourceIdOnce() ?: return@withLock
+                setStarLocked(type, id, starredValue(sourceId, type, id) == null)
+            }
+        }
+    }
+
     suspend fun setStar(
         type: StarType,
         id: String,
         starred: Boolean,
-    ): Result<Unit> =
-        starLock.withLock {
-            val sourceId = sourceRepository.activeSourceIdOnce()
-            val source = sourceRepository.activeMusicSource()
-            if (sourceId == null || source == null) {
-                return@withLock Result.failure(IllegalStateException("No active server"))
-            }
-            val previous = starredValue(sourceId, type, id)
-            updateStarred(sourceId, type, id, if (starred) System.currentTimeMillis() else null)
-            try {
-                source.setStar(type, id, starred)
-                Result.success(Unit)
-            } catch (cancellation: CancellationException) {
-                throw cancellation
-            } catch (failure: Exception) {
-                withContext(NonCancellable) { updateStarred(sourceId, type, id, previous) }
-                showMessage("Could not update star")
-                Result.failure(failure)
-            }
+    ): Result<Unit> = starLock.withLock { setStarLocked(type, id, starred) }
+
+    private suspend fun setStarLocked(
+        type: StarType,
+        id: String,
+        starred: Boolean,
+    ): Result<Unit> {
+        val sourceId = sourceRepository.activeSourceIdOnce()
+        val source = sourceRepository.activeMusicSource()
+        if (sourceId == null || source == null) {
+            return Result.failure(IllegalStateException("No active server"))
         }
+        val previous = starredValue(sourceId, type, id)
+        updateStarred(sourceId, type, id, if (starred) System.currentTimeMillis() else null)
+        return try {
+            source.setStar(type, id, starred)
+            Result.success(Unit)
+        } catch (cancellation: CancellationException) {
+            throw cancellation
+        } catch (failure: Exception) {
+            withContext(NonCancellable) { updateStarred(sourceId, type, id, previous) }
+            showMessage("Could not update star")
+            Result.failure(failure)
+        }
+    }
 
     private suspend fun starredValue(
         sourceId: Long,

app/src/main/kotlin/com/subtracks/playback/PlaybackService.kt

@@ -147,12 +147,13 @@ class PlaybackService : MediaSessionService() {
     private fun observeStar(songId: String?) {
         starJob?.cancel()
         currentSongId = songId
+        starred = false
         val library = library ?: return
         if (songId == null) {
-            starred = false
             session?.setMediaButtonPreferences(emptyList())
             return
         }
+        session?.setMediaButtonPreferences(listOf(starButton(false)))
         starJob =
             scope.launch {
                 val sourceId = sourceRepository?.activeSourceIdOnce() ?: return@launch
@@ -167,7 +168,7 @@ class PlaybackService : MediaSessionService() {
     private fun toggleStar() {
         val songId = currentSongId ?: return
         val library = library ?: return
-        library.star(StarType.Song, songId, !starred)
+        library.toggleStar(StarType.Song, songId)
     }
 
     private fun starButton(isStarred: Boolean): CommandButton =

app/src/main/kotlin/com/subtracks/ui/library/SongsContent.kt

@@ -1,5 +1,6 @@
 package com.subtracks.ui.library
 
+import android.os.Build
 import androidx.compose.foundation.Canvas
 import androidx.compose.foundation.border
 import androidx.compose.foundation.layout.Arrangement
@@ -174,7 +175,9 @@ private fun CoverArtCell(
             colorFilter = if (isPlaying) DIM else null,
         )
         if (isPlaying) {
-            Box(Modifier.matchParentSize().drawBehind { drawInnerShadow() })
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
+                Box(Modifier.matchParentSize().drawBehind { drawInnerShadow() })
+            }
             Box(
                 Modifier
                     .matchParentSize()
@@ -227,13 +230,27 @@ private fun PlayIndicator(
             val scale = minOf(size.width / PLAY_ARROW.viewportWidth, size.height / PLAY_ARROW.viewportHeight)
             withTransform({ scale(scale, scale, pivot = Offset.Zero) }) {
                 drawIntoCanvas { canvas ->
-                    val paint =
+                    val path = PLAY_ARROW_PATH.asAndroidPath()
+                    val glyph =
                         android.graphics.Paint().apply {
                             isAntiAlias = true
                             color = tint.toArgb()
-                            setShadowLayer(radius, 0f, 0f, PLAY_SHADOW.toArgb())
                         }
-                    canvas.nativeCanvas.drawPath(PLAY_ARROW_PATH.asAndroidPath(), paint)
+                    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
+                        glyph.setShadowLayer(radius, 0f, 0f, PLAY_SHADOW.toArgb())
+                        canvas.nativeCanvas.drawPath(path, glyph)
+                    } else {
+                        val halo =
+                            android.graphics.Paint().apply {
+                                isAntiAlias = true
+                                color = PLAY_SHADOW.toArgb()
+                            }
+                        canvas.nativeCanvas.save()
+                        canvas.nativeCanvas.translate(1f, 1.5f)
+                        canvas.nativeCanvas.drawPath(path, halo)
+                        canvas.nativeCanvas.restore()
+                        canvas.nativeCanvas.drawPath(path, glyph)
+                    }
                 }
             }
         }